In recent years, a display apparatus or a lighting apparatus using organic electroluminescence elements as organic light emitting elements have been developed. Such an organic EL element is typically manufactured by sequentially stacking a transparent electrode as an anode, an organic layer, and a metal electrode as a cathode on a transparent supporting substrate such as a glass substrate, a transparent plastic film, etc.
EL emission is made when electrons supplied from the cathode and holes supplied from the anode are recombined in the organic layer by a voltage applied between the transparent electrode and the metal electrode and excitons generated by the recombination transition from an excited state into a ground state. The EL emission light penetrates through the transparent electrode and is extracted from the transparent supporting substrate to the outside.
In recent years, for the purpose of obtaining high luminance, there has been proposed a structure called Multi Photon Emission (“MPE”), in which a plurality of organic EL layers is stacked with charge generating layers interposed therebetween.
If an organic EL device (hereinafter, the term “organic EL Device” is also used interchangeably to refer to an “OLED (Organic Light Emitting Diode)”) is used in illumination, such a MPE structure is suitable for improvements in luminance and durability.
In organic EL devices in the related art, conditions on luminance measurement have been set depending on light emitted out of a substrate (for the sake of convenience, referred to front luminance).
In general, though light distribution in the substrate has a forward projecting shape in order to reduce total internal reflection components, this does not necessarily maximize its total amount of light (total light flux).
Therefore, such organic EL devices in the related art have problems with effective extraction of light generated in an organic layer from the substrate.